My 2001 Expy EB with 5.4L engine was leaking gear oil 3 weeks ago when I was out of town. I took it to a local mechanic who said he changed the oil and replaced the pinion seal. After about 300 miles, I heard a roaring sound from the rear end, and when I checked the oil, it was low (could barely reach it with my little finger).

I decided to remove the oil rather than top it off, and I found that it was black colored and had some tiny flecks of varnish looking stuff in it. I now know that I should have removed the diff cover and cleaned it out, but I just refilled it with synthetic oil plus an additive. It didn't help the noise at all, and I believe the mechanic took this woman for a ride.

The noise sounds like an airplane engine winding down, and it only occurs when I take my foot off the accelerator. At least, I can't hear it if it occurs while I'm accelerating. I've never worked on a differential, and after reading everything I can find, it's beyond my expertise.

Am I right in assuming this is a problem with the differential?
How much should I expect to pay in the Houston area?
How do I find a reputable shop to do the work?

Thank you for getting back to me. I just got back from taking the truck to a drivetrain shop, and they said the mechanic probably set the preload too high, but they will call me this afternoon to let me know.

The mechanic said he used the 75-140 synthetic, but I'm beginning to think he didn't change the fluid but probably topped it off.